If you lost your Administrator membergroup and privileges due to a mistake or conversion, you will need to run the appropriate query for your SMF version.
The below queries will add you back as an Administrator on an SMF forum.
[vers=1.1]
Run this query in phpMyAdmin:
UPDATE smf_members SET ID_GROUP = '1' WHERE memberName = 'yourusername'[/vers][vers=2]
Run this query in phpMyAdmin phpMyAdmin:
UPDATE smf_members SET id_group = '1' WHERE member_name = 'yourusername'Alternatively, SMF 2.0 allows you to reattribute posts through the Forum Maintenance section of your Administration Center (Administration Center > Forum Maintenance > Reattribute User Posts). Simply fill in the form using the email or username of the old user, and the username for the user who is to have those posts. This only works for guest posts (remaining posts from deleted members are considered guest posts).
[/vers]

I lost admin account, what can I do?

If you do not have access to phpMyAdmin, the following instructions would help, This applies to SMF 1.1.x only:
Create a new .php file, copy and paste the fol. code in it, replace the "yourusername" variable with the new account's login name.
<?php
//Here we include SSI for globals used within the sql statement
include_once('SSI.php');
global $db_prefix;
$adminsquirt = db_query("UPDATE {$db_prefix}members
SET ID_GROUP = '1' WHERE memberName = 'yourusername'",__FILE__, __LINE__);
?>
Save the PHP file, upload it to your board directory (where SMF files reside on the server), and access it via URL.
-[n3rve]